236 PHYSICS. 



which may be mentioned those on " The limits of accuracy in measure- 

 ments with the telescope and the microscope" {Proc. Am. Acad., II, vi, 

 168), on '' Standard measures of length," and " On two forms of com- 

 parators for measures of length " (Am. Q. Mic. J., January and April, 1879), 

 and " On the present state of the question of standards of length." — 

 {Proc. Am. Acad., II, vii, 273.) More recently this physicist has de- 

 scribed the construction and early performances of a new diffraction 

 ruling-engine, which promises most valuable and important results. — 

 {Am. J. ScL, III, xix, 54, January, 1880.) Hilgard has i^ublished, under 

 date of July 10, 1880, an admirable report upon the methods and results 

 of a comparison of the American and British standards of length, in- 

 tended to form Appendix 12 to the Coast Survey Report for 1877. The 

 final conclusions with reference to the American standards are that 

 bronze No. 11 is standard at 62^.25 F., iron No. 57 at 62^.10 F., and the 

 mean yard of Troughton scale at 590.62 F. 



With reference to the molecular constitution of matter, Sir William 

 Thomson has continued his researches upon the " Vortex atom theory," 

 and has communicated to the Royal Society of Edinburgh papers upon 

 " Vortex statics " and upon " Vibrations of a columnar vortex," in which 

 the treatment is mathematical. — {Phil. Mag., V, x, 97, August, 1880.) Tol- 

 ver Preston has discussed the i)hysical asi)ects of the vortex atom theory, 

 with a view to reconcile the proved indestructibility of the atom with its 

 capacity for executing vibrations. He illustrates the motion of the material 

 forming the vortex atom by supposing a cylinder of India rubber to be 

 rotated about its longitudinal axis, and then, while still rotating, to be 

 bent into the form of a ring and the ends joined. Such a portion of 

 moving matter, mathematics has proved to have all the qualities of a 

 solid. It is elastic, and is competent to execute vibrations of definite 

 periods. Moreover, it is incapable of being divided or severed by the 

 colHsions of other similar atoms. In short, such vortex atoms are elastic, 

 indestructible bodies, capable of rebounding from each other without 

 loss of energy, and of executing vibrations of fixed periods. — {Nature, xxii, 

 56, 1880). 



Rlihlmann, from considerations founded on Avogadro's law, has given 

 some calculations on the dimensions of molecules. The radius of the 

 sphere of action for the nitrogen molecule he gives as 34x10-^ centi- 

 meter; for the carbon dioxide molecule, 16x10-^ centimeter; and for 

 the hydrogen molecule, 41x10"^ centimeter. Since one cubic centi- 

 meter at 0<=^ and 760"^™ pressure contains nearly 100 trillions of molecules, 

 it follows that the molecules themselves fill only about the three-thou- 

 sandth part of the entire volume which the gas occupies. The author 

 gives 15 X 10 ~ 23 gram as the absolute weight of a hydrogen molecule, and 

 360 as the specific weight. — {Beihl. Ann. Phys. Chem., No. 2, 57, 1879.) 

 Hodges has also given a computation of the size of molecules, based on 

 the work required to convert water into steam, this conversion being 

 considered as a breaking up of the water into a. large number of small 



